1. Field of Invention
The present invention relates to a canister for a vehicle and a fuel evaporative system provided with the same, and more particularly to a canister for a vehicle and a fuel evaporative system provided with the same which is applied mainly to a vehicle in which an operation (purge) region of an engine is reduced (i.e., hybrid vehicle) and which can reduce generation of evaporation gas in spite of small purge amount.
2. Description of Related Art
The automotive industry has actively sought to reduce pollutants in emissions. Exhaust gas is largely divided into tail pipe emission flowing out to the atmosphere after engine combustion and evaporative emission flowing out to the atmosphere due to evaporation of gasoline at a fuel system of a vehicle such as a fuel tank. One method for improving evaporative emission is using a canister.
Generally, gasoline includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility C8 to C10 hydrocarbons. Such gasoline is filled in a fuel tank. However, when the temperature of the surroundings is high or vapor pressure in the fuel tank is increased by movement of the vapor, fuel vapor leaks through crevices of the fuel tank. To prevent leakage of the fuel vapor, the fuel vapor is vented into a canister when the vapor pressure in the fuel tank is increased.
The canister has absorbent material (e.g., charcoal) for absorbing the fuel vapor from the fuel tank storing volatility fuel. If the hydrocarbons HC absorbed by the canister are vented into the atmosphere, the engine does not meet exhaust gas regulations. Therefore, an engine control unit operates a purge control solenoid valve in order to vent the hydrocarbons absorbed by the canister into the engine.
The evaporation gas is physically or chemically absorbed at the charcoal of the canister.
Physical absorption means that the evaporation gas is absorbed to the charcoal by Van der Waal's force that acts between molecules. Since physical absorption in which electron transfer between an adsorbate and an absorbent does not occur is reversible reaction, release may be easy, absorbing speed may be fast, and the physical absorption occurs well at low temperature.
Chemical absorption occurs by sharing electrons between the adsorbate and the absorbent. Since chemical absorption is non-reversible reaction, release may not be easy and absorbing speed may be slow.
Both of chemical absorption and physical absorption are exothermic reaction.
The evaporation gas absorbed at the charcoal is released by air supplied to the canister. Since release reaction is endothermic reaction, the release reaction occurs better at higher air temperature.
The evaporation gas absorbed at the charcoal of the canister can be diffused to the atmosphere. If canister temperature rises, C4 and C5 which are low molecular materials among ingredients of the evaporation gas absorbed at the charcoal near an evaporation gas supply passage are diffused to near an air passage and are absorbed at the charcoal near the air passage. After that, if the canister temperature rises again, low molecular materials absorbed at the charcoal near the air passage are leaked through the air passage. These phenomena are called bleed emission.
Meanwhile, a hybrid vehicle is provided with an engine outputting power by combustion of fuel and a motor outputting power of a battery. Recently, uses of the engine are declining for enhancement of fuel economy, and accordingly it is also declining that the fuel vapor of the canister is released and is burned again. Since the fuel vapor absorbed in the canister increases but the fuel vapor purged from the engine decreases, overflow of the fuel vapor may occur.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.